Move Solid's createMutable to new mutable package

[thetarnav]

This copies the current implementation of createMutable in solid 1.7.12 one-to-one, as the primitive is planned to be removed in core in future releases.
As the API is still useful to many people, it will be continued to be developed and maintained here.
The v1 of this package should remain the same as it was in solid, to avoid breaking existing apps, but later it can be extended past the current implementation, as it won't be "held back" by the tight compatibility with createStore.
createMutable in solid was some internal logic from createStore implementation, so using both createMutable and createStore in your app will mean larger bundle size after the move.

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[thetarnav]

@atk I needed to remove it so that both solid-js/store and solid-js/web resolve to the dev runtime version. Instead of the store resolving to prod, and web to dev, causing isDev to be true while the dev features were missing.